In a multi-phase brushless DC motor, the rotary field is established by a set of permanent magnets secured to the rotor and the stationary field is established by sequentially energizing a set of phase windings secured on the stator. A position or EMF sensing device detects the position of the rotor and a logic circuit energizes the stator windings relative to the detected position of the rotor to accelerate the rotor in the clockwise (CW) or counterclockwise (CCW) direction of rotation.
The motor phase windings may be energized with either bidirectional current (via a full wave bridge) or unidirectional current (via a half wave bridge) depending on the speed and torque characteristics desired for the particular application. With a full wave bridge two or more of the windings are always active (energized); this maximizes the motor output torque for driving relatively high loads, but at a relatively low motor speed. With a half wave bridge, the applied voltage is greater and the number of active windings at any point in time is reduced, as compared to the full wave example; this maximizes the motor speed for driving relatively light loads where high motor output torque is not required.